Such a measuring arrangement is known and is described for instance in the standard DIN 2271 where air under pressure is employed in place of the liquid used in the present arrangement. A similar arrangement is described in the patent document EP-B-0 109 936 (=U.S. Pat. No. 4,538,449).
As is seen in FIG. 1, the arrangement of the cited document consists in feeding via a pressure regulator 1 a pneumatic circuit comprising a first branch 15 provided with an inlet nozzle 16 and a measuring plunger 4 adapted for instance to be introduced into a bore 5 the diameter of which is to be measured, as well as a second branch 19 provided with an inlet nozzle 20 opening into a reference nozzle 21, the flow through which may be regulated. A pressure transducer 22 connected between branches 15 and 19 furnishes an electrical signal representing the difference of the pressures prevailing in such branches, which signal is amplified by an amplifying arrangement 10 prior to being led to an indicating arrangement 11 for instance. It is thus clear that if the plunger 4 is introduced into a larger bore, for instance, the pressure prevailing in branch 15 will be reduced and the variation of the pressure differential detected by the transducer 22 will be representative of the increase in diameter of bore 5. The essential interest of the differential assembly is to absorb automatically noise variations of pressure due, for instance, to the functioning of the pressure regulator 1 or to temperature effects.
The object of the invention described in patent document EP 0 109 936 is that of proposing an improvement in the known arrangements which permits considerable reduction in the measurement noise as well as the time constant of the measuring arrangement and sensitivity to vibration, while enabling an adjustment which is simple and independent of the sensitivity of the arrangement and of the null point of the output signal.
To this end, the cited invention provides the use of a differential pressure transducer using semi-conductor elements and arranging the assembly of the transducer and the pneumatic circuit within the measuring plunger. While leaving one at liberty to define the pneumatic circuit in an optimum manner so as to limit as far as possible the production of measurement noise, such solution enables the obtaining of a measuring arrangement entirely insensitive to vibrations and the time constant of which, on the order of a tenth of a second, is sufficiently small to enable obtaining extremely rapid measurements, for instance between two penetrations of an oscillating machine tool into a workpiece or measuring discontinuous surfaces of a workpiece in motion.
In spite of the improvements which it brings, the system of the cited invention gives rise to several difficulties, all coupled to the fact that the fluid employed is air under pressure:
The cutting liquid employed for cooling and lubricating the workpiece to be machined is often found to be a mixture based on petrol. If air under pressure is employed for the measurement of the dimensions of the workpiece at the same time as this is being machined, the cooling petrol may be pulverized by the air and thus bring about explosions should the cutting tool produce sparks. Generally as well, the cutting liquid thus pulverized may be harmful for the health or at least provoke general discomfort and dirtying of objects located in the surroundings of the machine.
It has been indicated that the time constant of the measuring arrangement is on the order of a tenth of a second (100 ms). This reaction time is connected to the fact that the fluid employed is air, basically compressible. In certain cases, the attained time of 100 ms slows the machining process, for instance in the machining of the truing of interior bores where the coming and going of the grinder could be much more frequent within a predetermined time lapse if the dwell time of the measuring arrangement within the piece could be shortened. Because of the elasticity of the air employed for the measurement, the pressure requires a certain time to be established and to be stabilized at the places of measurement which thus prevents rapid machining since it is necessary to proceed with a measurement which lasts a relatively long time following each passage of the tool.
It will be further noted that in the cited document the time constant of 100 ms has been capable of attainment thanks to the fact that the measurement arrangement and the measuring plunger form only a single part, thus diminishing the length of the paths travelled by the air. This advantage leads however to the providing of a heavier and larger apparatus which is poorly suited to the measurement of small workpieces.
The cited difficulties have already been anticipated by the applicant of this invention who has proposed, in an arrangement called Hydrocompar, the replacement of the air in the known systems by a liquid to overcome said difficulties.
By employing a liquid in the place of air, it has been possible to reduce considerably the time constant of the system and times on the order of 10 ms have been observed, which improves by 10 times the system response relative to systems formed according to the document cited hereinabove. This is due principally to the almost total incompressibility of the liquid. The liquid employed, as well as no longer being followed by pulverization effects, no longer requires construction of a clumsy and sometimes inconvenient compact system.
The Hydrocompar arrangement cited hereinabove uses input nozzles identical to those employed with arrangements employing air, and this has the disadvantage of creating undesirable turbulence in the liquid. Such turbulences bring about substantial background noise onto the pressure transducer.